Revise the return type of #getLabel

Address the feedback of API review, and change the return type
to CharSequence.

Bug: 255592779
Test: atest CtsContentTestCases:PreapprovalDetailsTest
Test: atest CtsPackageInstallTestCases:PreapprovalInstallTest
Change-Id: I777d54e8b9614f87c8a899c49ea476792ca8c515
3 files changed