Switch usages of legacy-test to android.test.base

The legacy-test library has been deprecated in favour of the
android.test.base library which is part of the SDK.

The following change descriptions were generated automatically and so
may be a little repetitive. They are provided to give the reviewer
enough information to check the comments match what has actually been
changed and check the reasoning behind the changes.

* core/tests/overlaytests/OverlayAppFiltered/Android.mk
    Removed legacy-test from LOCAL_JAVA_LIBRARIES because the source
    for module 'com.android.overlaytest.filtered_app_overlay' does not
    depend on classes from legacy-test

* core/tests/overlaytests/OverlayAppFirst/Android.mk
    Removed legacy-test from LOCAL_JAVA_LIBRARIES because the source
    for module 'com.android.overlaytest.first_app_overlay' does not
    depend on classes from legacy-test

* core/tests/overlaytests/OverlayAppSecond/Android.mk
    Removed legacy-test from LOCAL_JAVA_LIBRARIES because the source
    for module 'com.android.overlaytest.second_app_overlay' does not
    depend on classes from legacy-test

* core/tests/overlaytests/OverlayTest/Android.mk
    Replaced 'legacy-test' with 'android.test.base' in
    LOCAL_JAVA_LIBRARIES because the source for module 'OverlayTest'
    depends on classes from android.test.base

* core/tests/overlaytests/OverlayTestOverlay/Android.mk
    Removed legacy-test from LOCAL_JAVA_LIBRARIES because the source
    for module 'com.android.overlaytest.overlay' does not depend on
    classes from legacy-test

* test-mock/Android.mk
    Removed legacy-test from LOCAL_JAVA_LIBRARIES because the source
    for module 'repackaged.android.test.mock' does not depend on
    classes from legacy-test

Bug: 30188076
Test: make checkbuild
Change-Id: I729412304884cf5da88b05dbe8fbe645ad7b5b15