Restore file truncation where expected.

Several years ago ParcelFileDescriptor.parseMode() was fixed to match
the behavior of fopen(), since developers expect consistent behavior
between managed and native code.  FileUtilsTest.testTranslateMode()
verifies that all these modes are correctly translated.

However, this unintentionally changed the behavior of
ContentResolver.openOutputStream(), which only sends the 'w' mode
to the remote process.  Developers expect this API to behave like
the FileOutputStream constructor, which always truncates the file
unless opened with the append mode.

Since some remote providers may not be prepared to handle the 't'
mode, this change carefully uses Os.ftruncate() to restore this
expected behavior in all cases.

For other APIs that return opened files, this strategy is applied
to restore the original behavior, but only when the target SDK of
the app is expecting this truncation to take place.  The reason for
this is that moving forward our goal should always enable
ContentInterface APIs to be a transparent conversation between apps
without attempting to alter the behavior.  Apps talking with older
providers can apply the Os.ftruncate() logic themselves, if
desired, once they target Android Q or higher.

Bug: 157888856, 180680924
Test: atest CtsContentTestCases:ContentResolverTest
Change-Id: Iacec49164c4ce3891db0270635e9f458dea7becd