Add isChildDocument to MtpDocumentsProvider.

Bug: 32687127
Test: adb shell am instrument -w -e class com.android.mtp.MtpDocumentsProviderTest#testIsChildDocument com.android.mtp.tests/com.android.mtp.TestResultInstrumentation
Change-Id: I44ded3c73443735229a8b3465122f7e10c4da178
2 files changed