Change dynamic MIME feature accodring to API review

Remove PackageManager#clearMimeGroup method. Add expected best
practices of the MIME group naming to Javadocs.
getMimeGroup/setMimeGroup throw in case of undefined MIME group.

Bug: 151101912
Test: atest CtsDynamicMimeHostTestCases
Change-Id: Ica468c77785cb0f9862b412a0bbc664f23e2f2d4