Do BlockGuard check on Runtime.gc().

This adds an onExplicitGc method to BlockGuard.Policy and calls it in
Runtime.gc().

Bug: 3400644
Test: cts-tradefed run cts-dev -m CtsLibcoreTestCases
Test: cts-tradefed run cts-dev -m CtsOsTestCases
Change-Id: Ifa171a34cbd1cbde289a94da46b9c404f7bd506f
Merged-In: I82a37e38a4c8dbdf82cd988eccc2ed5d578e7ec0
6 files changed