Do not use default initializer for union

The next Clang update complains about this code pattern. Rewrite default
constructor to explicitly initialize the union field instead of using
the default initializer.

Test: Build
Bug: 37752547
Change-Id: I22a2aa392d7f4803282baed832b0fa2f852016ac
(cherry picked from commit 4fdbdd1692a549a31382f33f5d529fd141453ea1)
diff --git a/libs/math/include/math/half.h b/libs/math/include/math/half.h
index 615b840..7682973 100644
--- a/libs/math/include/math/half.h
+++ b/libs/math/include/math/half.h
@@ -56,8 +56,8 @@
  */
 class half {
     struct fp16 {
-        uint16_t bits = 0;
-        fp16() noexcept = default;
+        uint16_t bits;
+        explicit constexpr fp16() noexcept : bits(0) { }
         explicit constexpr fp16(uint16_t b) noexcept : bits(b) { }
         void setS(unsigned int s) noexcept { bits = uint16_t((bits & 0x7FFF) | (s<<15)); }
         void setE(unsigned int s) noexcept { bits = uint16_t((bits & 0xE3FF) | (s<<10)); }
@@ -68,11 +68,11 @@
     };
     struct fp32 {
         union {
-            uint32_t bits = 0;
+            uint32_t bits;
             float fp;
         };
-        fp32() noexcept = default;
-        explicit constexpr fp32(float f) : fp(f) { }
+        explicit constexpr fp32() noexcept : bits(0) { }
+        explicit constexpr fp32(float f) noexcept : fp(f) { }
         void setS(unsigned int s) noexcept { bits = uint32_t((bits & 0x7FFFFFFF) | (s<<31)); }
         void setE(unsigned int s) noexcept { bits = uint32_t((bits & 0x807FFFFF) | (s<<23)); }
         void setM(unsigned int s) noexcept { bits = uint32_t((bits & 0xFF800000) | (s<< 0)); }