Make BaseAppSecurityTest abstract.

This class has no @Test methods, which causes CTS to fail if it tries to run it.

Test: m -j CtsAppSecurityHostTestCases CtsDocumentClient && \
      cts-tradefed run commandAndExit cts-dev -m CtsAppSecurityHostTestCases \
      -t android.appsecurity.cts.ScopedDirectoryAccessTest
Bug: 69981990

Change-Id: I401260580a2997687d8d779c9491c2b7b7398373
1 file changed